Polyimides generally have superior heat resistance, mechanical properties and electrical characteristics compared to other general-purpose resins and engineering plastics, finding wide applications as molding materials, composite materials, electrical and electronics materials, optical materials, etc. In particular, polyimide/metal laminates, which have a metal foil and a polyimide layer and used as flexible printed boards and the like, require low warpage. However, polyimides generally have a higher coefficient of thermal expansion (CTE) than metals, which may contribute to the occurrence of warpage in polyimide/metal laminates.
Inorganic glass, a transparent material, is widely used as a panel substrate or other parts in display industries, including liquid crystal display and organic EL display. However, inorganic glass is increasingly suffering from the limitations of high specific gravity (weight), fragility, low flexibility, and so forth. Under this circumstance, research is being made on transparent materials that can replace glass. Plastic films made of transparent, heat-resistant resin have the advantages of reducing weight, imparting impact resistance, and good moldability. Moreover, there is an increasing expectation that the use of such plastic films makes it possible the development of flexible displays, which is very difficult with inorganic glass.
Polyimides have been reported that have cyclohexanediamine (CHDA) as a diamine unit (see Patent Literatures 1 to 4). Polyimides with a cyclohexanediamine unit may have the feature of low coefficient of thermal expansion (CTE) (see, e.g., Example 3 in Patent Literature 1). In addition, since the diamine unit has alicyclic structure, these polyimides have much higher transparency than those in which all diamine units have aromatic structure. However, since cyclohexanediamine is generally expensive, the compound is desired to be used in combination with other diamine such as norbornene diamine (NBDA) (see, e.g., claim 6 in Patent Literature 2) for cost reduction.
A technique for preparing a block polyimide by chemical imidization is known (see, e.g., Non-Patent Literature 1). Chemical imidization is a process whereby imidization is effected using such an imidization agent as acetic anhydride or pyridine. Generally, chemical imidization is a complex process that requires purification. In the case of thermal imidization, on the other hand, imidization can proceed by adding an azeotropic solvent such as xylene to a polyamic acid varnish (polymer solution of polyimide precursor) and heating the solution. However, thermal imidization is not a common method for preparing block polyimides.